Question

Treatment of an aqueous solution of \(\mathrm{NiCl}_{2}\) with \(\mathrm{H}_{2} \mathrm{NCHPhCHPhNH}_{2}\) gives a blue complex \(\left(\mu_{\mathrm{eff}}=3.30 \mu_{\mathrm{B}}\right)\) which loses \(\mathrm{H}_{2} \mathrm{O}\) on heating to form a yellow, diamagnetic compound. Suggest explanations for these observations and comment on possible isomerism in the yellow species.

Short answer

The blue complex is likely octahedral/high-spin; upon heating, it becomes a square planar/low-spin yellow complex, suggesting isomerism potential like cis-trans forms in the yellow species.

Step-by-step solution

Analyze Formation of the Blue Complex

The initial treatment of the aqueous solution of \(\mathrm{NiCl}_{2}\) with the ligand \(\mathrm{H}_{2} \mathrm{NCHPhCHPhNH}_{2}\) forms a coordination complex. The blue color and the magnetic moment \(\mu_{\text{eff}} = 3.30 \mu_{\text{B}}\) suggest the presence of unpaired electrons, indicating that the nickel is in the +2 oxidation state and likely forming a high-spin configuration. This configuration fits with a tetrahedral or octahedral geometry around the nickel.

Analyze the Yellow Compound Formation

Upon heating, the blue complex loses \(\mathrm{H}_{2} \mathrm{O}\) to form a yellow, diamagnetic compound. The change in color and magnetic properties suggest a change in geometry or spin state. The diamagnetic nature indicates all electrons are paired, suggesting the yellow compound could be a low-spin square planar complex with nickel in the +2 state.

Assessing Isomerism in the Yellow Compound

Considering the square planar geometry of the yellow complex, there could be cis-trans isomerism. In a cis-isomer, similar ligands are adjacent, while in a trans-isomer, they are opposite. This could affect color and reactivity, contributing to different properties and potential applications of the yellow species. However, the stability may favor one isomer over the other.

Key concepts

Magnetic Properties

Magnetic properties in coordination complexes arise due to the presence of unpaired electrons. In the context of the provided exercise, the blue complex has a magnetic moment of \(\mu_{\text{eff}} = 3.30 \mu_{\text{B}}\), indicating unpaired electrons in the nickel center.

• The presence of unpaired electrons generally results in a paramagnetic substance, which is attracted to magnetic fields.
• The magnetic moment is indicative of the number of unpaired electrons. In this case, it aligns with a high-spin Ni(II) complex, suggesting several unpaired spins.

When heated, the complex turns yellow and becomes diamagnetic. Diamagnetism occurs when all electrons are paired, resulting in a substance that is not attracted to magnetic fields. This transformation suggests a change from a high-spin to a low-spin state, likely due to a structural change in the nickel complex.

Geometric Isomerism

Geometric isomerism in coordination compounds is showcased through the possible cis and trans forms. This type of isomerism arises from different spatial arrangements of ligands around the central metal atom.

• In the yellow nickel complex discussed, which is likely to adopt a square planar geometry, geometric isomerism can play a role.
• In a cis-isomer, ligands that are identical are adjacent to each other. This can affect the compound’s color and reactivity.
• Alternatively, in a trans-isomer, similar ligands are opposite, potentially leading to different physical and chemical properties.

The effectiveness of the ligand arrangement in stabilizing the complex might be favored by either isomer, impacting its use in applications and contributing to distinct characteristics.

Transition Metal Chemistry

Transition metal chemistry involves elements that can form various coordination complexes. These metals are characterized by their ability to exist in multiple oxidation states and to form colored compounds.

• Nickel, like many transition metals, can accept electrons to form complex geometries with different ligands, showcasing distinct colors due to d-d electron transitions.
• These transitions are responsible for the vivid colors observed in many complexes, as electrons jump between energy levels when light is absorbed.
• Formation of complexes with transition metals often involves hybridization, where atomic orbitals mix to form new hybrid orbitals suitable for bonding.

The dynamic chemistry of transition metals such as nickel is evident in the switch from blue to yellow upon heating. This entails a change in the light absorption patterns due to structural rearrangements and ligand interactions.

Oxidation States

Oxidation states are crucial in understanding coordination chemistry, as they determine the electron counts around a metal center. Nickel, in these complexes, generally appears in the +2 oxidation state.

• In the initial blue complex, nickel is likely in a +2 state, and this aligns with a typical high-spin d\(^8\) configuration.
• The blue color arises from specific electron arrangements and the corresponding magnetic properties, allowing some electrons to remain unpaired.
• Upon heating, when the complex becomes yellow and diamagnetic, some structural changes leading to a square planar arrangement might reduce the direct oxidative role of nickel, yet remains in the same +2 oxidation state.

Understanding how oxidation states influence the electron configuration and geometry of coordination complexes can provide insight into the properties and behavior of these substances under different conditions.